False-colour transmission electron micrograph of erythroblasts, primitive red blood cells, in human bone marrow. Erythropoiesis, the formation of mature red cells, is directed towards the production of a cell without organelles such as a nucleus; instead it is packed with haemoglobin. These primitve cells still show the nuclei as dominant oval shapes surrounded by pink cytoplasm. An original stem cell differentiates to form a proerythroblast, a large nucleated cell with no haemoglobin. The increase in haemoglobin proceeds with a gradual loss of all organelles. Magnification: x 5000 at 6x7cm size.